The potential advantages of an epitope based vaccine are as follows. First, epitope based vaccines represent a powerful approach to induce a specific immune response against the selected epitope (s), avoiding the side effects of other unfavorable epitopes in the complete antigen . Second, epitope based vaccine has other considerable advantages, including increased safety, the opportunity to engineer the epitopes rationally for increased potency and breadth, and the ability to focus the immune response on conserved epitopes . Moreover, the epitope based vaccine approach has been shown to be successful in various infectious diseases, such as Neisseria meningitides infection, and tuberculosis . In this study, an epitope based vaccine was proposed as a possible candidate vaccine with a remarkable ability to induce protection against HSV-2 infection.
B cell antigenic epitopes from HSV-2 glycoproteins were screened using three software algorithms: Biosun, DNAstar, and Antheprot algorithms. The common results from software were used as candidate epitopes of B cell epitopes. In this study, six B cell epitopes (gB2466-473, gC2216-223, gD26-18, gE2483-491, gG2572-579 and gI2286-295) were predicted and identified as epitopes (data no shown), which have not been reported. CD4+ T cell epitopes were predicted successively by the APC software algorithm, the lysosome hydrolysis cleavage site software algorithm and the TAP software algorithm. The results of the APC software algorithm provided candidate peptides for the lysosome hydrolysis cleavage site software algorithm. Similarly, the results of the lysosome hydrolysis cleavage site software algorithm provided candidate peptides for the TAP bind software. Some CD4+ T cell immunodominant epitopes, which had higher affinity to TAP than ICP47, were screened as candidate CD4+ T cell epitopes . Four of them (gD221-28, gD2205-224, gD2245-259 and gB2162-177) were identified as epitopes (data no shown), which have not been reported. Here, epitope-prediction program SYFPEITHI, NetMHC and MHCPred were used to analyze the sequence of gD2 according to their different algorithms. Two CD8+ T cell immunodominant epitopes, gD210-20 and gD2268-276, were predicted and identified as epitopes (data no shown) , which have not been reported.
The gD2, gB2, gC2, gE2, gG2 and gI2 of HSV-2 are largely exposed structural glycoprotein, which have B-cell and T-cell epitopes and are major immunogenic antigen . Ideally, epitope based vaccines should contain both B cell epitopes, which are essential for the protective antibody response, and T cell epitope (CTL epitopes, Th epitopes) that will serve to induce CTL and Th immune responses. Therefore, in the present study, we selected six B cell epitopes (gB2466-473, gC2216-223, gD26-18, gE2483-491, gG2572-579 and gI2286-295), four CD4+ T cell epitopes (gD221-28, gD2205-224, gD2245-259 and gB2162-177) and two CD8+ T cell epitopes (gD210-20 and gD2268-276) from HSV-2 glycoproteins to construct an epitope based vaccine (MEAP). These confirmed epitopes were all inserted into the extracellular fragment (1-290) of HSV-2 glycoprotein D to construct multi-epitope assembly peptides (MEAPs) by replacing some non-epitope amino acid sequences. Three-dimensional structures of the 14 MEAPs were predicted by the software algorithms of Accelrys Discovery Studio and Moe2008 in order to screen a MEAP. All epitopes, especially the B cell epitopes, independently displayed on the surface of the MEAP [12, 27, 28].
HSV-2 specific IgG response was detected in the sera of mice immunized with MEAP after three boosts, as well as in the mice immunized with inactivated vaccine. This suggests that the MEAP protein and inactivated vaccine were both capable of inducing a significant humoral response in mice. Studies have demonstrated that neutralizing antibodies are important for protection against HSV-2 infection and titers ≥ 1:10 are considered to be indicative of protective immunity . The mean neutralization titer in the mice immunized with MEAP was 1208 after the third boost, and these titers were higher than those induced by the inactivated vaccine (P < 0.05). Importantly, these antibody titers were sufficient to protect mice against HSV-2 infection, which indicates the potential of MEAP as a vaccine against HSV-2.
In addition, Cell-mediated immunity plays an important role in efficient protection. The results of cytokine assay and CTL activity indicated that CTL activity and the level of Th2-type cytokines (IL-4) produced in the culture supernatant from mice vaccinated with MEAP was significantly higher than in the groups given inactivated vaccine and PBS. This demonstrated that the multi-epitope vaccine, MEAP, constructed in this study stimulated intensive cellular immunoreaction. Specific CTL are critical in the recovery from infection and the clearance of HSV-2, and the Th2 immune response against HSV-2 is also protective , the remarkably increased CTL activity and IL-4 production in the MEAP group may indicated that the MEAP has a higher degree of protection effect.
Challenge experiments showed that the mice that received the MEAP or the inactivated vaccine were completely protected against HSV-2 infection, even though the mean neutralizing antibody titer before challenge of the group vaccinated with the MEAP was lower than that of the group given the inactivated vaccine. This high level of protection, also, is usually related to the MEAP specific neutralizing antibodies . However, we observed high levels of CTL activity in the splenocytes of the mice immunized with the MEAP. This may be contributed to the high levels of protection in the mice immunized with the MEAP, because CTL activity by the T cells was found also to be an essential determinant of protective immunity against HSV-2 .